In a dynamic microphone, a voice coil fixed to a diaphragm is disposed in a magnetic gap defined by magnetic circuit components. In the dynamic microphone, the voice coil vibrates together with the diaphragm in the magnetic gap in response to received sound waves to generate audio signals in response to the vibration velocity due to electromagnetic conversion between the voice coil and a magnetic field. The voice coil then outputs the audio signals. At this time, if the magnetic circuit is affected by some external factor to vary a magnetic flux in the magnetic gap, the voice coil generates signals independent of sound waves. The signals independent of sound waves are noise.
A factor generating noise in the dynamic microphone is, for example, an alternating magnetic field applied to the magnetic circuit. When the alternating magnetic field is generated from a commercial AC power supply, the resulting noise has a relatively low frequency corresponding to the frequency of the commercial AC power. The noise caused by the commercial AC power supply is called hum noise.
Hum noise flows into a microphone through various paths. For example, hum noise is generated by contact of a finger with a switch knob for the switching operation of a microphone. Japanese Unexamined Patent Application Publication No. 2010-68364 discloses a switch knob connected through a metal wire to a shield cover of a switch body. This configuration eliminates electrostatic coupling between the switch knob and a switch contact point to prevent hum noise from occurring at contact of a finger with the switch knob.
Japanese Unexamined Patent Application Publication No. 2009-200869 discloses a narrow-directivity condenser microphone for reducing, for example, hum noise. In more detail, this configuration includes an acoustic tube in electrical contact with a built-in microphone unit through an electrically conductive intermediate disposed therebetween.
Both the above patents disclose condenser microphones for reducing hum noise. In general, dynamic microphones employ electromagnetic conversion as described above and thus generate larger hum noise than condenser microphones.
Dynamic microphones illustrated in FIGS. 4 and 5 are proposed in order to cancel hum noise based on unique configurations utilizing such characteristics of the dynamic microphones. Techniques related thereto will be described below.
FIG. 4 illustrates a unit frame 101 which is a base of a microphone unit. The substantially cylindrical unit frame 101 composed of magnetic material functions as an outer yoke. A disk yoke 102 is fixed in the center hole of the unit frame 101. A disk magnet 103 is fixed above the yoke 102. A disk pole piece 104 is fixed above the magnet 103. The yoke 102, the magnet 103, and the pole piece 104 are bonded to each other. While the unit frame 101 functions as an outer yoke as described above, the yoke 102 functions as an inner yoke.
The outer circumferential surface of the yoke 102 is in close contact with the inner circumferential surface of the unit frame 101. A round gap is provided between the outer circumferential surface of the pole piece 104 and the inner circumferential surface of the unit frame 101. A magnetic circuit is defined by the yoke 102, the unit frame 101, the round gap, and the pole piece 104. A magnetic flux from the magnet 103 returns to the magnet 103 through the magnetic circuit. In this configuration, the round gap functions as a magnetic gap.
A cylindrical member 135 is fixed to the upper outer circumference of the unit frame 101. The cylindrical member 135 has an inward flange 136 at the upper inner circumference. The flange 136 is fixed to the upper outer circumference of the unit frame 101. The flange 136 has multiple through holes 137 in the vertical direction. A cylindrical space between the inner circumferential surface of the cylindrical member 135 and the outer circumferential surface of the unit frame 101 communicates with a space above the cylindrical member 135 through the through holes 137.
The circumference of a diaphragm 105 is fixed to the upper end of the cylindrical member 135. The diaphragm 105 is made by shaping a material such as synthetic resin or metal. The diaphragm 105 includes a center dome and a sub-dome surrounding the center dome. The outer circumference of the sub-dome is fixed to the outer circumference of the cylindrical member 135. The diaphragm 105 can vibrate in response to the sound pressure from received sound waves, in the anteroposterior direction (the vertical direction in FIG. 4) around the outer circumference of the sub-dome as a supporting node.
A voice coil 106 is fixed along the boundary between the center dome and the sub-dome in the diaphragm 105. The voice coil 106 has an air-cored cylindrical shape formed by winding a thin conductive wire, one end of the cylindrical shape being fixed to the diaphragm 105. The voice coil 106 is disposed in the magnetic gap while the outer circumference of the sub-dome in the diaphragm 105 is fixed as described above. The sub-dome in the diaphragm 105 covers the through holes 137 of the cylindrical member 135 from above.
Adjacent to the reverse of the diaphragm 105 (below the diaphragm 105 in FIG. 4), a protector 107 is fixed to the top surface of the pole piece 104. A constant gap is provided between the domal top surface of the protector 107 and the center dome of the diaphragm 105. The protector 107 has a center hole 171 communicating with center holes 141, 131, and 121 of the pole piece 104, the magnet 103, and the yoke 102, respectively.
Adjacent to the obverse of the diaphragm 105, a resonator 108 has an outer circumference fixed to the upper outer circumference of the cylindrical member 135. A constant gap is provided between the central domal ceiling surface of the resonator 108 and the center dome of the diaphragm 105. The resonator 108 has a center hole 181 for introducing external sound waves to the diaphragm 105. The resonator 108 additionally has multiple holes 182 around the center hole 181. A lid 110 is fit to the lower end of the unit frame 101. A lower opening of the unit frame 101 is closed by the lid 110 to define a relatively large air chamber 111.
The diaphragm 105 vibrates in the anteroposterior direction in response to a variation in the sound pressure from received sound waves. The voice coil 106 also vibrates in the anteroposterior direction together with the diaphragm 105. The voice coil 106 vibrates in the magnetic flux passing through the magnetic gap and outputs audio signals in response to the variation in the sound pressure. The dynamic microphone illustrated in FIG. 4 electro-acoustically converts the signals as described above to output the audio signals from both ends of the voice coil 6 to an external device.
The dynamic microphone illustrated in FIG. 4 further includes a noise canceling coil 151 for canceling hum noise flowing thereinto from the exterior. The noise canceling coil 151 is an air-cored coil. The noise canceling coil 151 is fixed so as to be wound on the outer circumferential surface of the resonator 108 fixed to the front end of the cylindrical member 135.
Another example dynamic microphone illustrated in FIG. 5 includes a noise canceling coil 152 fixed to the obverse of a resonator 108. This dynamic microphone has the same configuration as that of the microphone unit illustrated in FIG. 4 except for the arrangement and size of the noise canceling coil 152. The noise canceling coil 152 composed of an air-cored coil has a central axis coaxial with that of the voice coil 106. The noise canceling coil 152 is also provided in order to cancel hum noise flowing from the exterior into the dynamic microphone unit.
The noise canceling coils 151 and 152 illustrated in FIGS. 4 and 5 are connected in series to the voice coil 106 and wound in the direction of canceling hum noise in the voice coil 106 induced by an alternating magnetic field flowing into the microphone.
Next, explanations will be given on undesirable vibratory noise in a dynamic microphone and an example dynamic microphone having a mechanism for eliminating the vibratory noise. In a dynamic microphone, a voice coil has large inertia force due to its large mass and therefore generates vibratory noise in response to mechanical vibration applied from the exterior. In particular, a first-order pressure-gradient microphone employs a mass control for controlling a bidirectional component. A diaphragm is therefore designed so as to have a resonant frequency lower than a main sound acquisition band. As a result, large vibratory noise is generated at the resonant frequency of the diaphragm.
Such vibratory noise can be cancelled by, for example, a dynamic microphone described in Japanese Examined Patent Application No. 61-30800. This patent discloses a vibration detecting device in a microphone case. According to the disclosure, the vibration detecting device detects noise signals outputted from a microphone unit due to mechanical vibration applied from the exterior, converts the detected signal into the opposite phase, and outputs the converted signals. The detected signals with the opposite phase are then added to the output signals of the microphone unit to eliminate the noise signals. The vibration detecting device can be referred to as a vibration pickup. The vibration detecting device includes, for example, a canceling coil in, for example, a microphone unit case and a magnet fixed to the inner surface of a microphone case so as to face the canceling coil, the magnet generating a magnetic field around the canceling coil. The microphone case receives unnecessary external vibration to cause vibration of the microphone unit due to its inertia force relative to the microphone case. This generates electromotive force in the canceling coil. The invention utilizes this electromotive force as signals for canceling the noise signals.